It is known that dihydro-1,3,2-benzodiazaphosphole-2-oxides may be prepared by refluxing an addition product of phenylphosphonic dichloride with ortho-diamines in bromobenzene (J. Org. Chem., 26, 3995 (1961)). This method of preparation, however, has certain drawbacks. For example, ortho-diamines are difficult to make and are relatively unstable. Also, the preparative technique utilizing bromobenzene is tedious and often results in poor yields of the desired dihydro-1,3,2-benzodiazaphosphole-2-oxide product.
It was thought that a synthesis involving intramolecular cyclization of nitrenes derived from 2-nitrodiphenylamine derivatives would yield phenazines. Nitrenes are generally obtained by a loss of nitrogen from an azide or by the removal of oxygen from nitroso or nitro compounds. Cyclization of nitrenes derived from 2-nitrophenyl sulfides has led to the formation of phenothiazines. The analogous reaction of a 2-nitro-diphenylamine derivative with excess trialkyl phosphite did not yield the phenazine. Instead, an alternative nitrogen to phosphorous ring closure was achieved leading to the formation of dihydro-1,3,2-benzodiazaphosphole-2-oxide in high yield. This method of preparing dihydro-1,3,2-benzodiazaphosphole-2-oxides is much more efficient than methods previously used.